Not ideal to have the ally hull as the negative. Great way to speed up corrosion on the hull.
Best way to stop it is to run a negative wire to a bus bar and tidy all the wiring up. Blue seas do some decent ones.

From memory most chargers will do this. I remember when I use to work in our marine division at work when charging batteries we use to disconnect both the positive and negative terminals.

When I park up my boat at home I always disconnect both terminals to also be on the safe side

Cheers

Hang your cock on the gunnel and enjoy the experience :)

 Lol yep dale comment of the week ,  

 Simplest explanation is that the earth on the charger is a different potential to the general mass of earth you are standing on. This is not uncommon, particularly with stuff that uses switch mode recitification, although it can happen with any gear. Google iPhone earths and be prepared to be terrified--our electrical inspector has measured the metal frames of iPhones on charge against an independant earth, ie, the type you would be standing on outside, or maybe on a slab that doesn't have the reo earthed, and measured in excess of 150v. This is how they kill people who talk on it while on the charger. Disconnecting both terminals from the battery will fix it in most cases, but, if the charger has a metal case and is touching your ali hull, the condition can still exist and be more than a tingle. Shouldn't be a problem with glass boats, unless sitting on damp carpet or something conductive.

edit--I'll give you are more thorough explanation of this, including how you get "tingles" ( or fatal shocks) from your taps at home, if you want, or just leave it at that.

Thanks for the heads up, sorry to come in on the thread but after reading all the posts about it, it has given me a bit of a shake up about charging my batteries in my ali boat.

I do have electrical experience as I am a fridgie by trade. 

I decided to put a meter across the ali hull and an earth from the point directly next to it and i'm getting 60+ AC volts. So ultimately the hull is live when on the charger. Apparently chargers with a 3 pin plug dont suffer from this issue. My other option is to completely disconnect the batteries when charging.

I will be contacting the charger manufacturer and seeing what they have to say.

 Under normal circumstances, there "shouldn't" be an issue if the charger has a PE( protective earth) connected via a 3-pin plug. But, if your home mains connection is less than optimal ( neutral connection is developing an impedance, very common, most common cause of "tingles" through taps), you will still get it. or an actual fault in the charger itself. If you have it on the end of a long extension lead, and the charger goes faulty to earth, you may not generate enough fault current to trip your protective device, although if you have the circuit RCD protected that should cover it. But don't rely on them. How long since you have done the manual trip test on your RCD's? So many people just never do, so, when the time comes in a fault situation, they just don't work.

A 60v potential to an independent earth, such as you have just measured, could generate sufficient current to  kill a child. BUT, and this is a big but, such measurements should be taken with a Low Impedance meter. Most good quality multimeters are High Impedance on voltage measurement, so as to not disturb circuit conditions when you try to measure the voltage across two points. When you are doing  electric shock investigation, a Low Impedance voltmeter must be used, which gives a more accurate reading of the voltage which a human would detect across the same two points. So, your good quality multimeter may show 60VAC, but across the same points, a Low Impedance meter may only show, say, 20V AC. This will probably  not generate sufficient current to kill a small child, but may certainly be enough to make them not want to touch daddies boat.  The little tacker, if he/she has wet hands and feet, standing on wet ground, could present as low as 500 ohms impedance, which, with 20V AC applied, could give you a 40mA current flow. Which could kill them. Dry hands/feet/dry shoes on, dry ground, could easily be over 1000 ohms, so, halve the current flow.

I've put some words in italics /bold, just to emphathise these are very broad guidelines--if in doubt, please go for the safest approach every time.

 The whole basis of our standard, above ground electrical safety is the MEN system. And it is being steadily degraded by the increasing use of poly underground instead of steel. So nearby metallic buried services carry it instead, as in your excellent example.

You raise another good point too, about "one day a tingle, the next day far worse."  The bad street neutral connection I mentioned before is a very common cause, but not the only one. I've done a comprehensive 5 day electrical shock investigation course, on top of my Nominees training, so I believe I've got a fairly good handle on the basics. I have also delivered training on this stuff, so you work out how to best explain it.

Anyway, as part of this training , we were walked through actual events. The most tragic this investigator told us of was one where there were uncomfortable tingles coming through the shower in a home. This bloke had three little girls, and the tingles would come and go. It was investigated by a sparkie, who could find no fault. The dad wasn't convinced, although the tingles seemd to be gone. So he stood there and ran the shower for a while, seemed OK. So he left, and one of his little girls got in the shower, turned it on--an immediate  fatal shock.  The fault turned out to be in the electric HWS--the element was failing to earth. Because the testing had been coincidentally done with the thermostat off, there was no apparent fault. When dad ran the shower for a while, tank started filling with cold water. When the little girl turned the shower on, thermostat had kicked in, and the element insulation failed completely--and she died. 

So, don't dismiss "tingles", and don't let people tell you they are just static. They certainly can be, but don't take them lightly. Western Power don't--if you ring up and report, they will be there as quickly as they can, and I don't mean next week-hopefully within hours.

A lot of sparkies don't really understand this stuff, unfortunately. I used to get on the whiteboard while delivering the Electrical Competency training for Rio sparkies--earth fault loop impedance wasn't part of the course, but once I started doing it, i realised just how few really understood it.  You can draw it up really simply on a whiteboard, and you would see the lightbulb come on above their heads. That is when delivering training is personally rewarding.

 Reminds me of an overnighter where I stuck my hand into the livewell to grab a slimey and wondered why I was getting tingles.  Quickly worked out the overflow outlet was blocked and the water was sloshing over the top and into the area where we had the fridge running.

All good and laugh about it now but a reminder to be carefull with such things. The slimeys did not fair too well.